Maurice's Stratēgikon. Handbook of Byzantine Military Strategy, translated from Greek to English by George T. Dennis (Philadelphia, 1984). Wikipedia has an entry on this valuable work (Strategikon of Maurice) attributed to Byzantine emperor Maurice (582-602). In eleven books it describes: cavalry battle formation, ambushes, baggage trains, tactics and drills, surprise attacks, sieges, characteristics and tactics of various peoples; mixed formations of infantry and cavalry, and discusses related topics such as camps and hunting. Internet Archive has other copies of this work. The present edition is bookmarked and searchable. Also available at Internet Archive is the Greek text with facing German translation (1981), and other works by the translator/author. Wikipedia has an entry on Byzantine Military Manuals, of which this is an important example.

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Available as a separate pdf download is the author's translation and study of the Taktika of Byzantine emperor Leo VI (ruled 886-912), Corpus Fontium Historiae Byzantinae volume 49 (2000). A Wikipedia entry describes it here.

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Available as a separate pdf download is the author's Three Byzantine Military Treatises (1985), CFHB volume 25, which contains one work from the 6th century and two from the late-10th century.